Power semiconductor products are often fabricated using extended-drain N or P channel MOS transistors, where current is to be switched at high voltages. These drain-extended devices offer high current handling capabilities and are able to withstand large blocking voltages without suffering voltage breakdown failure. Accordingly, such transistors are ideally suited for power switching applications, particularly where inductive loads are to be driven. N-type drain-extended MOS devices (DENMOS transistors) are asymmetrical devices in which a p-type channel region is typically formed in a p-well between an n-type source and an extended n-type drain. Low n-type doping on the drain side provides a large depletion layer able to withstand high blocking voltages.
Depletion NMOS transistor devices have a threshold voltage (Vt) that is less than zero (e.g., negative Vt), whereby the channel is conductive when Vgs is zero. Depletion transistors are sometimes used in power management startup circuits and other applications in which a constant low on-state current is needed with no gate biasing. N-channel depletion devices allow conduction between the transistor drain and the source when a positive drain voltage is applied without having to positively bias the gate, and thus are desirable for creating current sources or resistive-type loads (e.g., such as a pull-up load for NMOS inverters) in power management or other types of circuits. In many applications, the depletion MOS device gate is connected to the source so that Vgs=0, and the transistor is always on (e.g., Vgs>Vt). In this configuration, the operation of the depletion MOS is analogous to a resistance, wherein the effective resistance is generally proportional to the ratio of the transistor length and width.
In certain power management circuit applications, current sources are needed for providing a current in the presence of relatively high drain voltages. For example, 30 volts or more may be provided to the depletion MOS drain terminal in mixed signal device power conditioning circuitry, wherein startup circuitry requires a current source without any gate bias voltage. However, conventional depletion MOS devices are not well suited for operation with such high drain voltages. In particular, such devices typically suffer from poor breakdown voltage ratings (e.g., the drain-to-source voltage at which breakdown occurs, BVdss), where breakdown voltage is often measured as drain-to-source breakdown voltage with the gate and source shorted together. Accordingly, there remains a need for improved depletion MOS transistor devices capable of operating with high drain voltages, as well as manufacturing techniques for fabricating the same.